The activated sludge process is a biological process for the treatment of waste water. Advanced processes comprise a sequence of oxic, anoxic and anaerobic process phases in mixed liquor suspended solids that correspond to metabolic processes of living organisms. The process phases can be controlled by controlling the oxygen available in the waste water.
Dissolved oxygen is present in the oxic phase. Carbonaceous compounds are oxidized. Ammonium is oxidized to nitrates and nitrides. The organisms take up phosphates from the waste water for storage in an energy rich form as polyphosphates.
The anoxic phase is characterized by the absence of dissolved oxygen. The organisms use chemically bound oxygen in the form of nitrates and nitrides for respiration. Nitrates and nitrides are converted to nitrogen and carbonaceous compounds are oxidized.
In the anaerobic phase, no dissolved oxygen is present and the organisms have used the available chemically bound oxygen. The organisms metabolize the previously stored energy rich form of phosphate and release phosphate back into the waste water as orthophosphate.
Phosphates are removed from waste water by this process because more orthophosphate is taken up by the organisms in the oxic phase and converted to polyphosphate than is released in the anaerobic phase. The process is cyclic so the greater the release of orthophosphate in the anaerobic phase, the greater the orthophosphate uptake in the oxic phase. Therefore, removal of phosphates from waste water can be increased by increasing the amount of phosphate released during the anaerobic phase.
The process disclosed in U.S. Pat. No. 5,076,928 is an advanced waste water treatment process in which oxic, anoxic, and anaerobic process phases are controlled in a single activated sludge tank. The process phases are controlled in response to the measured turbidity of a clarified water sample of mixed liquor suspended solids. Aeration is switched on at a maximum turbidity set point to initiate the oxic phase and switched off at a minimum turbidity set point to initiate an anoxic phase that is followed by the anaerobic phase. Aeration is switched back on to initiate the oxic phase when the turbidity increases again to its maximum set point.
The process described above provides an undisturbed anaerobic process phase for biological phosphate elimination. The anaerobic process phase provides a high phosphate release to achieve an intensive phosphate uptake in the subsequent oxic phase. Metallic precipitants can be added as an adjunctive chemical treatment for greater phosphate removal.
However, operation of activated sludge treatment facilities using advanced waste water treatment as described above has not proved altogether satisfactory. Consumption of metallic precipitants becomes significant when phosphate release in the anaerobic phase is too great. Also, phosphate concentrations in the effluent of the final clarifier of the waste water treatment plant can increase.
Accordingly, it is an object of the invention claimed herein to create a process for biological waste water treatment that avoids high phosphate concentrations in the effluent of the waste water treatment plant and, at the same time, reduces the consumption of metallic precipitants substantially to zero.